This invention relates to compositions comprising a polymercaptan which has at least two mercaptan (--SH) groups and a compound which has both at least two, preferably at least three, ethylenically-unsaturated double bonds and at least one free carboxyl group. It also relates to the polymerisation of such compositions by means of actinic radiation and/or heat-activated free-radical catalysts, and to the use of the polymerised products as surface coatings, as adhesives, and in reinforced composites, but particularly in printed circuits.
In U.S. Pat. No. 4,220,513 there are described compositions comprising
(1) a compound containing in the same molecule both at least one 1,2-epoxide group and at least two groups chosen from allyl, methallyl, and 1-propenyl groups, e.g., 2,2-bis(3-allyl-4-(glycidyloxy)phenyl)propane and bis(3-(1-propenyl)-4-glycidyloxy)phenyl)methane, and
(2) a compound containing at least two mercaptan groups attached to aliphatic carbon atoms per molecule, e.g., pentaerythritol tetrathioglycollate.
The compositions are caused to polymerise by the reaction of mercaptan groups with the allyl, methallyl, or 1-propenyl groups under the influence of actinic irradiation or free-radical catalysts.
Such compositions which further contain
(3) a heat-curing crosslinking agent for polyepoxides are also described; the polymerised products, containing more than one epoxide group per average molecule, could thus be subsequently crosslinked in situ.
Published British Patent Application No. 2,012,781 A discloses similar compositions, in which component (1) contains at least one phenolic hydroxyl group instead of at least one epoxide group, e.g., 2,2-bis(3-allyl-4-hydroxyphenyl)propane and bis(3-(1-propenyl)-4-hydroxyphenyl)methane, component (3) being correspondingly a heat-activated crosslinking agent for phenolaldehyde novolac resins. The polymerised products, containing more than one phenolic hydroxyl group per average molecule, could likewise be subsequently crosslinked in situ by heating.
We have now found that certain compounds containing acrylate or methacrylate ester groups can be caused to polymerise very rapidly by exposure to actinic radiation or to the action of free-radical catalysts in the presence of certain mercaptans to form products having valuable properties. The acrylate or methacrylate esters employed also contain allyl, methallyl, and/or 1-propenyl groups. It is believed, although the usefulness of this invention does not depend on the truth of this belief, that the allylic type double bonds react preferentially with the mercaptan groups, thus facilitating the polymer-forming processes taking place by means of the double bonds in the acrylic or methacrylic groups. Whatever the mechanisms of the reactions may be, it has been found that the compositions polymerise very rapidly to form surface coatings and adhesive bonds.
In the so-called negative resist method for producing printed circuits, a substrate is coated with a photopolymerisable composition and the coating is exposed to actinic radiation through an image-bearing transparency with substantially transparent and substantially opaque areas so that polymerisation takes place in the areas of the coating which have been struck by radiation. Alternatively, such exposure can be carried out using computer-controlled scanning laser beams instead of such a transparency. Then the image is "developed" by dissolving away with a suitable solvent the areas where polymerisation had not occurred, i.e., those areas which had not been struck by actinic radiation. Subsequently, the substrate, usually of copper, may be exposed to etching liquids so that the areas not protected by the remaining portions of the composition are etched. In further processing, such as in the manufacture of multilayer boards, the layer of composition remaining may have to be removed by mechanical means.
Disadvantages of process presently employed are that organic solvents are often used for development, with associated risks due to toxicity and flammability, and that mechanical processes for removing the photopolymerised composition may cause damage to the printed circuit.
As already indicated, the esters also contain carboxylic acid groups; if exposure to actinic radiation is effected as through a negative with substantially opaque areas, it is possible to remove the composition from unexposed areas by treatment with an aqueous alkaline solution, due to salt formation on the carboxylic acid groups. The use of an organic solvent for this purpose, with its attendant disadvantages of toxicity and/or flammability, is therefore avoided. A further advantage is that the polymerised composition can be subsequently removed if desired by use of more strongly basic aqueous solutions.
U.S. Pat. No. 4,120,721 describes radiation-curable compositions for use in coating and image-forming processes. The compositions comprise a polyunsaturated acrylic or methacrylic-terminated urethane, a vinyl monomer which is preferably an ester of acrylic or methacrylic acid, a polymercaptan containing at least two --SH groups per molecule, and a photoinitiator. To obtain the acrylic or methacrylic-terminated polyurethane, a di-isocyanate is caused to react with an hydroxy-substituted acrylate or methacrylate in an amount such that one stoichiometric equivalent of free isocyanate groups remains in the reaction product, then these isocyanate groups are caused to react with the stoichiometric equivalent of hydroxyl groups in a polyhydric alcohol, preferably a polyethylene glycol or a polypropylene glycol. In addition to approximately fifty hydroxyalkyl acrylates and methacrylates listed as suitable are 2-hydroxy-3-(allyloxy)-propyl acrylate, 2-hydroxy-3-cinnamylpropyl acrylate, 2-hydroxy-3-(crotonyloxy)propyl acrylate, and the corresponding methacrylates. The polymers specifically described were made from a hydroxyalkyl acrylate or hydroxyalkyl methacrylate and lacked allylic-type double bonds. There is no suggestion that the use of acrylic or methacrylic-terminated urethanes derived from hydroxyalkyl acrylates which contain additional ethylenic unsaturation confers any advantage.
In British Pat. No. 1,474,715 there are described photopolymerisable acrylic esters containing free carboxylic acid groups which are prepared by the reaction of an at least dianhydride of a polycarboxylic acid with an alcohol containing at least two acrylic ester groups. In U.S. Pat. Nos. 4,079,183, 4,181,807, and 4,179,577 there are described photopolymerisable compounds containing free carboxylic acid groups and some specific unsaturated groups linking two aromatic rings, obtained by reaction of an at least dianhydride of a polycarboxylic acid with alcoholic hydroxyl groups of an alcohol containing at least one of the specified unsaturated groups. In all of these publications removal of the unphotopolymerised material by contact with an aqueous alkaline solution is described, but the photopolymerisation of such materials in the presence of a polymercaptan is not mentioned.